1. Field of the Invention
This invention relates to a developing device in an electrophotographic copying apparatus of the liquid developing type, and more particularly to a developing device in which masses of toner present in developing liquid composed of carrier liquid and toner are dispersed to thereby maintain the toner density constant.
2. Description of the Prior Art
In electrophotographic copying machines, the liquid development has been superior to the dry type development in that the former is more efficient and can produce better copies than the latter. The developing liquid used therefore is composed of carrier liquid and toner, and the toner must be maintained uniformly dispersed to provide good image reproduction. Further, in view of the gradual toner consumption caused by the developing process, it is also necessary during a long-time use to detect the toner density at all times to maintain the developing liquid at a density adapted for the image to be copied.
For these purposes, it has heretofore been the practice to measure the density of the developing liquid by using photoelectric detector means comprising a lamp and a light receiving element. However, the photoelectric detector means simply immersed in the liquid within a developer container as was done in the prior art would readily permit deposition of toner which in turn would cause density differences in different parts of the liquid. In addition, the toner would often adhere to a transparent member interposed between the lamp and the light receiving element, thereby reducing the detecting function of the detector means. Also, in another conventional arrangement wherein the photoelectric detector means is disposed outwardly of the developer container and the developing liquid is supplied by a pump, the developing liquid rarely stays in the detector means during the inoperative condition of the apparatus so that the surface of the transparent member tends to dry up and permits residual toner to adhere thereto. For these and other reasons, it has been difficult with such arrangement to ensure accurate density detection and to provide image reproduction of constant and excellent quality by simply detecting the density and supplying a predetermined amount of toner.
Furthermore, the developing liquid tends to evaporate during the inoperative condition of the apparatus to readily cause the toner in the carrier liquid to adhere and solidify. When the apparatus is restarted, such solidified toner may be admixed with a circulating flow of developing liquid which is supplied to developing electrode means, thus causing irregular image reproduction to take place or injuries to be imparted to the photosensitive medium.